1. Field of the Invention
This invention relates to a method of producing ceramics which can be sintered at a low temperature and have a high moldability, and more particularly to a method of producing zirconia ceramics of the above type.
2. Description of the Prior Art
Methods of producing zirconia ceramics according to the prior art use zirconia powder made by wet grinding, resulting in the following shortcomings. Namely, with the wet ground zirconia powder, high sintering temperatures in excess of 1,500.degree. C. are necessary, so that various difficulties associated with the high sintering temperature, such as a very low energy efficiency and a low mechanical strength due to large grain growth at the high sintering temperature, result.
To reduce the sintering tempeature, a method of using zirconia powder having a small grain size has been proposed, but this method has shortcomings of the low moldability of the zirconia powder and the large shrinkage which occurs during sintering. There have not been any methods which meet the requirement of both high moldability and low sintering temperature.
FIG. 1 shows a typical laminated oxygen sensor, which comprises a metallic layer 1 for a first electrode, hereinafter referred to as "first electrode", a solid electrolyte layer 2, a substrate 3, and a metallic layer 4 for a second electrode; hereinafter referred to as "second electrode". In a typical method of the prior art for producing the laminated structure of FIG. 1, the solid electrolyte layer 2 is formed by using a prefired zirconia layer made of a slurry containing wet ground zirconia powder, and the first electrode 1 is formed by using an electrode layer mainly consisting of electron-conductive metallic powder.
The green zirconia layer 2 is overlaid on the substrate 3 while inserting the electrode layer therebetween, which substrate 3 is either a green layer made of a ceramic material or a fired layer. After firing them into an unitary body, the second electrode 4 is formed on the solid electrolyte layer 2 by a physical method such as evaporation or a manual method such as brushing. Such method of the prior art has a shortcoming in that the sintering temperature of the green zirconia layer made of wet ground zirconia powder is high, being higher than 1,500.degree. C., so that the metallic particles of the first electrode 1 grow too large, resulting in a considerable reduction of the activity of the electrode.
To overcome this shortcoming, another method of the prior art reduces the grain size of the zirconia powder in the green zirconia layer for the solid electrolyte layer 2 by using wet grinding, so as to reduce the sintering temperature of the green zirconia layer. However, such method also has a shortcoming in that, the small grain size achieved by the wet grinding tends to cause cracks and pinholes in the green zirconia layer, and such cracks and pinholes remain even after the sintering of zirconia layer, and a solid electrolyte layer of sound and dense structure cannot be obtained.